Method and apparatus for a non-oil-filled towed array with a novel hydrophone design and uniform buoyancy technique

ABSTRACT

A structure and method for constructing a non-oil-filled towed array providing a single cable entry point for each channel regardless of the number of hydrophones used to make up a hydrophone group or array is described. The method and apparatus enables uniform buoyancy of the hydrophone array and the primary cable around which the hydrophone array is wound. Uniform buoyancy is achieved through the addition of hollow micro-spheres into a Reaction Injection Molded (RIM) polyurethane material used to mold the hydrophones. Additional buoyancy may be desired adjacent heavier cable sections where connectors and telemetry modules are located. An embodiment enables precise adjustment of hydrophone cable buoyancy by providing precise adjustment of the concentration of hollow glass micro-spheres in areas where more or less buoyancy is desired.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/635,031 filed on Aug. 4, 2000, now U.S. Pat. No. 6,498,769.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system and method for hydrophones forsensing acoustic pressure waves. In particular the invention relates toa technique for obtaining uniform buoyancy and increased sensitivityutilizing a hollow microsphere loaded reaction injection moldingpolyurethane material.

2. Description of the Related Art

Current methods of constructing non-oil-filled or solid towed arrayshave proven to be prohibitively expensive and complex due to theirmethod of construction, which adversely affects reliability. Suchtechniques are described in U.S. Pat. Nos. 5,774,423; 5,361,240;5,883,857; and 4,733,378. As a result, solid towed arrays, whiledemonstrating substantial advantages over oil filled arrays, have metwith limited acceptance within the seismic and surveillance communitiesdue to their high price and unreliability. Thus there is a need for atechnology which addresses the primary causes of high cost and lowreliability with a unique method of construction that eliminates theprimary cause of failure and reduces the labor required to construct thearray.

The primary cause of failure of solid towed arrays results from therequirement to make electrical connections for each of the numeroushydrophones that make up a single channel in an array. In the case ofthe cable described in U.S. Pat. No. 5,883,857, the method ofconstruction calls for a discrete entry into the primary cable in orderto connect each hydrophone. Prior methods of construction have utilizeda “floatation” cable design that extrudes a buoyant material, such asfoamed polyethylene, over an inner jacketed cable, which is then coveredwith an outer extrusion of polyurethane. The primary problem with thisdesign is the fact that there is no bond between the polyethylene foamand both the inner and outer jackets since they are dissimilarmaterials. This procedure results in an effective path way between theunbonded dissimilar materials enabling water to migrate up and down thelength of the cable in the path way when the outer protective shield ofthe cable is damaged or a leak occurs relative to the point of cableentry where the hydrophones are attached. Attempted solutions to thisproblem have proven to be unreliable.

Thus, there is a need for a method of constructing a solid ornon-oil-filled hydrophone which does not enable formation of a path wayfor water to migrate up and down the length of the cable when the outerprotective cable shield is damaged or a leak otherwise occurs and whichprovides a cost effective method and apparatus for a solid hydrophone.

SUMMARY OF THE INVENTION

Embodiments of non-oil-filled arrays, and embodiments of equipment andmethods of forming non-oil-filled arrays, are described herein. Anembodiment of an array provides minimization of cable entry points toone entry point for each channel irregardless of the number ofhydrophones used to make up a hydrophone group or array. An embodimentalso provides vastly superior protection of the primary cable bundlefrom damage over present known methods.

In an embodiment, the hydrophone array is provided with uniformbuoyancy. Uniform buoyancy is achieved through the use of a hollowmicro-sphere loaded Reaction Injection Molded (RIM) polyurethanematerial. Prior designs have relied on foaming in order incorporate airbubbles into molded areas in the hydrophone and hydrophone cable toachieve buoyancy. The result of prior methods, however, has been asignificant variation in the amount of buoyancy achieved. Prior methodshave been unable to precisely control the amount of buoyancy in areas ofthe cable where additional buoyancy is desired. For example, additionalbuoyancy is desired adjacent the approach to heavier sections of thehydrophone cable where connectors and in the case of digital arrays, thetelemetry modules are located.

In one embodiment, a method enables precise adjustment of hydrophonecable buoyancy by providing precise adjustment of the concentration ofhollow glass micro-spheres in areas where more or less buoyancy isdesired. Use of an Reaction Injection Molding (RIM) process, results insignificant reductions in cost and time in the construction of solidtowed arrays. The RIM material enables use of a softer, lower durometermatrix in which the micro-spheres reside resulting in a hydrophone cablethat is more flexibility than prior known construction methods. Forexample, the prior known methods provide a minimum bending radius of 6feet, whereas an embodiment of the method and apparatus provides aminimum bending radius of 18 inches.

An embodiment also provides greater protection of the interiorhydrophone cable due to the homogeneous nature of the RIM material(which eliminates the necessity of layering of dissimilar materials andthe creation of potential path ways for water to run up and down thehydrophone cable as discussed above) which carries hydrophone signalsand array telemetry, resulting in an improvement in the reliability ofthe system and greater immunity from damage due to abrasions or damagefrom other sources, for example, shark bite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of the flexible diaphragm and rigid back planeof the present invention;

FIG. 2 is a cross-sectional view of the hydrophone of the presentinvention showing the matching of the silver ink patterns with theconcave portions of the diaphragm;

FIG. 3 is a side view of the preferred hydrophone of the presentinvention prior to mounting the hydrophone on the cable;

FIG. 4 illustrates a cross-sectional representation of the rigid tubeand molded plastic back plane taken substantially along line 4-4 of FIG.2;

FIG. 5 a schematic representation of the piezo thin film element withpass through construction of the present invention;

FIG. 6 is an illustration of a cable having connector at both ends forconnection hydrophone sections together or connecting hydrophonesections to a transmission device such as a digital or analog telemetrymodel; and

FIG. 7 is a schematic representation of the molding of the hydrophoneonto a hydrophone cable which is then helically wound onto a primarycable.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

An embodiment of a hydrophone provides a polymer film, air backed benderdesign, that unlike previous designs will not reduce the circumferentiallength of the polymer film with an increase in hydrostatic pressure. Incertain embodiments, a method and apparatus provides a polymerhydrophone that exhibits minimal change in sensitivity with depth changeand provides a substantially unlimited crush depth.

Turning now to FIG. 1, an embodiment of diaphragm 10 and back plane 12are illustrated. Diaphragm 10 slides over back plane 12. Minimalsensitivity to depth change and substantially unlimited crush depth areprovided by a design comprising the hydrophone diaphragm 10 having aseries of concave faces 11 that run longitudinally along the length ofthe hydrophone diaphragm 10 and discretely around the circumference ofthe hydrophone diaphragm 12. In the example of FIG. 1, the eight concavesurfaces of diaphragm 10 occupy the eight octants around thecircumference of back plane 12. Each of the concave surfaces 11 forms adiscrete hydrophone bender element using one section of a continuouspiece of piezo film 20, shown in FIG. 5.

The back plane 12 of the hydrophone is molded onto a rigid tubularmember shown in FIG. 2, in such a manner as to create a cylindricalsurface 15 at each end of the back plane. The back plan is provided withlongitudinal ridges 13 residing upon the rigid tubular member 21, shownin FIG. 2. The molded back plane fits into the shape of the diaphragm 10to form a surface against which the flexible diaphragm 10 stops whensufficient hydrostatic pressure is applied. Stopping against the backplane enables stretching under hydrostatic pressure without stretchingthe film beyond its yield point. Mounting of the flexible diaphragm 10onto back plane 12 is illustrated in FIG. 2.

Turning now to FIG. 2, a cross section of an embodiment of a hydrophoneis illustrated. As shown in FIG. 2, diaphragm 10 slides over back plane12 and back plane ridges 13. Air voids 14 are formed between diaphragm10 and back plane cylindrical portion 15 and back plane ridges 13.Hydrophone cable orifice 18 is surrounded by rigid tube 21. Rigid backplane 12, shown in FIG. 1, resides on top of rigid tube 21. The entirestructure is surrounded with molding 16. FIG. 4 illustrate across-sectional view of rigid tube 21 and molded back plane 12 havingcylindrical portion 15 and ridges 13.

Turning now to FIG. 5, a thin film homo-polymer piezo element 20 isconstructed to enable wires to be connected to two sides of the film 20with a flexible conductive ink 22 applied to both sides of the polymerto form the piezo element 20. The front side of piezo element 20 isvisible in FIG. 5. The back side of piezo element 20 looks substantiallysimilar to the front side and is not shown. The front side of piezoelement 20 is coated with a conductive silver ink pattern 22, which isconnected to outside pins 24. The back side of piezo element 20 iscoated with a similar silver ink pattern and is connected to inside pins26. This construction technique enables multiple piezo elements to beconnected together in either a series or parallel configuration withoutthe necessity of wires to pass the signal across the piezo element 20.The silver ink pattern sections 22 are interspersed with voids 28.Connectivity across voids 28 between adjacent silver ink sections 22 onthe front side of piezo element 20 is provided by metallic traces 30.Similarly, connectivity across voids 28 between adjacent silver inksections 22 on the back side of piezo element 20 is provided by metallictraces 30.

As shown in FIG. 2, voids 28 are positioned on top of and aligned withback plane ridges 13 and silver ink portions 22 coincide with diaphragmconcave portions 11. This alignment provides for deformation of concavediaphragm portions 11 and silver ink sections 22 between ridges 13. Thearrangement of FIG. 2 reduces the effect of passive capacitancecontributions from ridge supported voids areas 28 of the piezo elementthat are not deformed by incident vibration. The piezo element 20 isbonded to the concave diaphragm 10 with a transfer adhesive such as3M-VHB using a press with a face having the same shape as concavesurface 11 of diaphragm 10 upon which the material 20 is adhered.

As shown in FIG. 2, back plane 12 provides ridges 13 runninglongitudinally down the length of the hydrophone, forming a series ofstandoffs that fit into the high points 17 of the diaphragm formed atthe joining edges of or space between adjacent concave surfaces 11. Thisconstruction serves to lock the high points 17 in place on back planeridges 13 and results in substantially all deformation of the film dueto dynamic pressure variation (sound) occurring between these fixedpoints at high point 17 which is coincident with void 28. The cumulativechange in length of the piezo film in some embodiments is greater thanthat of a simple cylinder resulting in improved sensitivity of thehydrophone for a given signal pressure level.

There are two methods of embodying the hydrophone for mounting on thecable. As shown in FIG. 3, the outer covering for the hydrophoneconsists of a cylindrical sleeve 16, one end of which has an end that isof the same diameter as the back plane and another end that is of alarger diameter to allow for the insertion of the backplane/diaphragm/film assembly. Once the back plane/diaphragm/filmassembly is inserted and wire connections made between pins 24, 26 andtwisted pair 38, the space between the back plane/diaphragm/filmassembly and the inner wall of the outer sleeve 16 is filled with asuitable potting material. The hydrophone assembly 40 now has a singletwisted pair of wires 38 entering one end of the hydrophone assembly andexiting the other end of the hydrophone assembly. Back plane 12 ismolded from a material that is bondably compatible with the materialthat is used to form the outer covering of the hydrophone.

Cylindrical portion 15 of the back plane 12 extends beyond the length ofthe sleeve to enable injection molding of end caps 14 on the hydrophone,sealing the hydrophone and the cable entry points. In the case of thisexample the molding step also is used to form two oppositely located“cups” 42, as shown in FIG. 3 on each end of the hydrophone. The cupshave an outer diameter equal to the maximum diameter of the outer sleeve16 formed on the ends of hydrophone segments 40.

As shown in FIG. 7, after locating hydrophone assemblies 40 in thedesired locations along a primary cable 50, buoyant material 52 isinjected along the length of the cable between the hydrophones andinside the cups to which the material bonds. The resulting assemblyforms a constant diameter non-oil-filled towed array of significantlyreduced diameter. The buoyancy of the hydrophone cable and the primarycable around which the hydrophone cable is helically wrapped, can beselectively controlled to increase or decrease the desired buoyancy at adesired point along the hydrophone cable length, within the hydrophonesor within the hydrophone cups by increasing or decreasing the hollowmicrosphere concentration accordingly. In an embodiment using RIM, afolia material is used in molding the hydrophone cups and forms abondable interface between the RIM materials used in the forming thehydrophone cups and the overmolding of the hydrophone cable.

An alternative embodiment provides for use of RIM material, withouthollow microspheres, to produce the molded hydrophone structuredescribed above and indicated in the FIGS. 1-7. Use of RIM materialproduces a lower cost hydrophone embodiment compared to the first methodand is accomplished with a single injection of RIM material.

As shown in FIG. 7, in certain embodiments, hydrophones are constructedalong a single strain relieved twisted pair cable 50 of relatively smalldiameter. Each hydrophone 40 is molded to enable longitudinalpass-through of the cable 50 to attached subsequent hydrophones within agroup. The length of the cable 50 between hydrophones is sufficient toenable winding the cable 50 helically around a primary cable bundle 60,providing strain relief during deformation of the cable while towing. Asingle entry point into the primary cable is made at the head of thehydrophone group where appropriate electrical connections are madebetween signal conductors in the primary cable and the twisted pair fromthe hydrophone group. The connections are made using standard take-outmethods.

Substantially uniform neutral buoyancy of the entire primary cablesection is accomplished with a syntactic Reaction Injection Moldedmaterial that provides high flexibility and consistent buoyancy thatwill not be reduced by over-pressurization. Control of the hollow sphereconcentration during Reaction Injection Molding of the hydrophone,associated hydrophone cable and primary cable is provided by a “ModeQuad Sure Shot 150” available from Hi Tech Engineering, Grand Rapids,Mich.

As shown in FIG. 6, in certain embodiments, a connector 70 can beattached on the end of each hydrophone section 58 to enable connectingthe hydrophone sections together or connecting a hydrophone section orsections to a transmission device such as a digital or analog telemetrymodule. Additional microspheres are added to the cable to increasebuoyancy at each end of the hydrophone section to compensate for theadditional weight of the connectors at the ends, thereby maintainingneutral buoyancy along the entire length of the section.

The foregoing description is intended as an example of a preferredembodiment and not intended to limit the scope of the invention, whichis defined by the following claims.

What is claimed is:
 1. A solid towed bender hydrophone comprising: adiaphragm having a tubular shape; a thin film piezo element attached tothe diaphragm; and a back plane having a cylindrical shape, furthercomprising at least one longitudinal rib on the exterior surface of theback plane wherein the back plane and the at least one rib slide intothe center of and slidably engage the tubular diaphragm.
 2. Theapparatus of claim 1 wherein the diaphragm further comprises a pluralityof discrete concave surfaces running longitudinally along the diaphragmwherein the at least one rib acts as an offset between the back planeand the diaphragm, each concave surface using a section of the piezofilm to form a discrete hydrophone bender element.
 3. The apparatus ofclaim 2 wherein the back plane and at least one rib are molded onto arigid tubular member.
 4. The apparatus of claim 2 wherein the back planehas a tubular shape having a hollow center through which a hydrophonecable passes.
 5. The apparatus of claim 1 wherein the continuous filmpiezo element has a front and back side and is constructed so that awire can be connected each side of the film with a flexible conductiveink.
 6. The apparatus of claim 2 wherein the thin film piezo elementfurther comprises a conductive ink pattern on each side of the film,wherein the pattern comprises a plurality of ink sections interspersedwith voids, wherein ink patterns adjacent a void are connected with aconductive strip across a void between adjacent ink patterns.
 7. Theapparatus of claim 6 further comprising a set of pins attached to thefront ink patterns and a set of pins attached to the back ink patternswhich enable connecting a plurality of hydrophones in series orparallel.
 8. The apparatus of claim 6 wherein the voids are positionedcoincident with the back plane ribs.
 9. The apparatus of claim 8 furthercomprising a reaction injection molding covering.
 10. The apparatus ofclaim 9 further comprising a molding cup formed on each end of theapparatus for receiving a hydrophone cable.
 11. The apparatus of claim10 further comprising a plurality of hydrophones physically attached endto end and wound helically around a primary cable.
 12. The apparatus ofclaim 11 wherein the plurality of hydrophones provide a singleattachment point to the primary cable.
 13. A method for making a solidtowed bender hydrophone comprising the steps of: providing a diaphragmhaving a tubular shape; providing a thin film piezo element attached tothe diaphragm; and providing a back plane having a cylindrical shape,further comprising at least one longitudinal rib on the exterior surfaceof the back plane wherein the back plane and the at least one rib slideinto the center of and slidably engage the tubular diaphragm.
 14. Themethod of claim 13 wherein the diaphragm further comprises a pluralityof discrete concave surfaces running longitudinally along the diaphragmwherein the at least one rib acts as an offset between the back planeand the diaphragm, each concave surface using a section of the piezofilm to form a discrete hydrophone bender element.
 15. The apparatus ofclaim 14 further comprising the step of molding the back plane and atleast one rib onto a rigid tubular member.
 16. The apparatus of claim 14wherein the back plane has a tubular shape having a hollow centerthrough which a hydrophone cable passes.
 17. The apparatus of claim 13further comprising the step of: connecting a wire to a back side of thefilm with a flexible conductive ink and connecting a wire to the frontside of the film with a flexible conductive ink.
 18. The method of claim14 further comprising the step of: placing a conductive ink pattern oneach side of the film, wherein the pattern comprises a plurality of inksections interspersed with voids; and connecting the ink patternsadjacent a void with a conductive strip across a void between adjacentink patterns.
 19. The method of claim 18 further comprising step of:connecting a set of pins to the front side ink patterns; connecting aset of pins to the back side ink patterns, thereby enabling connectionof a plurality of hydrophones in series or parallel.
 20. The apparatusof claim 18 further comprising the step of: positioning the voids arecoincident with the back plane ribs.
 21. The apparatus of claim 20further the steps of: covering the apparatus with a reaction injectionmolding material.
 22. The method of claim 21 further comprising the stepof: forming a molded cup on each end of the apparatus for receiving ahydrophone cable.
 23. The apparatus of claim 22 further comprising thestep of: winding a plurality of hydrophones physically attached end toend and helically around a primary cable.
 24. The method of claim 23further comprising the step of: attaching the plurality of hydrophonesto the primary cable at a single attachment point.